Silicon is the semiconductor most commonly used for integrated circuits, and silicon integrated circuit technology is presently extremely well developed. The dominant position of silicon in the integrated circuit markets is partially due to the fact that it is both abundant and relatively inexpensive as compared to other semiconductors such as Group III-V compound semiconductors. For example, silicon wafers or substrates are presently approximately in order of magnitude cheaper than are GaAs wafers. The integrated circuit or other device is often fabricated using an epitaxial layer of silicon which is grown on the silicon substrate although the circuit may be fabricated directly on the wafer.
However, for a variety of reasons, it is often desirable to have epitaxial semiconductors other than silicon on silicon substrates. In pursuing this direction, the objective is to enhance the range of useful devices which can be fabricated using silicon substrates. For example, some semiconductors, such as Group III-V compound semiconductors, may lead to devices in which the carrier mobility is higher than it is in silicon devices or they may make it possible to integrate optical functions with electronic functions on the same substrate. In the latter case, it is contemplated that Group IV, III-V, II-VI or other compound, as well as non-silicon elemental, semiconductors will be used to fabricate optical devices while the electronic functions will be performed either by devices fabricated in silicon or in the epitaxial layers of the non-silicon semiconductor. Compound semiconductors are preferred over silicon for the optical functions because they often have a direct bandgap, while silicon has an indirect bandgap, and have bandgaps which extend over a wide range of wavelengths. The latter feature permits fabrication of optical devices, e.g., lasers and photodetectors, which are useful over a broad range of wavelengths including the visible and the infrared. This approach to device fabrication thus combines the low cost, easy handling, ready avalability, etc., of silicon substrates as well as the mature Si very large scale integration technology with the desirable attributes offered by other semiconductors.
However, growth of high quality non-silicon semiconductors on silicon substrates is generally extremely difficult because the desired non-silicon semiconductors typically have lattice constants that differ significantly from that of silicon, and high quality epitaxial growth is thus difficult to obtain because of the lattice constant mismatch. When utilization of the heterointerface is not required, various intermediate layers may be grown provided that the top layers where the devies will be located are of the desired device quality. Although there are many pitfalls in the path of this development, some, e.g., such as the possible chemical incompatibility of the two materials or their different lattice symmetry, are not directly related to the lattice mismatch. The latter, which is the concern of this application, manifests itself adversely by generating misfit dislocations, which thread the epitaxial layers, thus degrading their quality.
One approach to increasing the number of semiconductors which may be grown with a high degree of crystalline perfection on a particular type of substrate is to use a strained layer superlattice between the substrate and the desired semiconductor. A strained layer superlattice consists of a plurality of interleaved layers having different compositions and lattice constants with the strain produced by the lattice constant mismatch between the two semiconductors being accommodated by distortion of the lattice rather than by generation of misfit dislocations. For example, a plurality of GaAs layers may be interleaved with a plurality of AlGaAs layers.
Strained layer superlattices may also be used in a different context which has been used successfully with, for example, GeSi superlattices grown on, for example, Si substrates. A compositionally graded layer may be grown between the substrate and the superlattice. The lattice constant of the compositionally graded layer varies from that of the substrate to that of the desired compound semiconductor. The misfit dislocations that arise because of the lattice mismatch between the compositionally graded layer and the substrate often have their propagation terminated in the superlattice. Although the reason for this behavior is not presently known with absolute certainty, it is likely to be associated with the additional strain introduced by the superlattice which makes the threading propagation of a dislocation unfavorable. Thus, the misfit dislocations generated by the compositionally graded layer are trapped in the superlattice, and homogeneous alloy layers grown above the superlattice may now be used as a substrate for the epitaxial growth of additional semiconductor layers which are lattice matched to those alloy layers rather than the substrate.
Although many combinations of semiconductors have been proposed for the strained layer superlattices, one combination of semiconductors that has not received any attention from those skilled in the art is the combination of tin and another Group IV semiconductor. The use of tin appears especially attractive because it has a lattice constant of 0.6489 nm as opposed to 0.5431 or 0.5646 nm for silicon or germanium, respectively. Such a large lattice constant difference opens the possibility of epitaxially growing many types of compound semiconductors on a SiSn or GeSn alloy layer grown on a silicon or germanium substrate after a superlattice has been used to trap the misfit dislocations. However, those skilled in the art have studiously avoided the use of such tin containing superlattices. It was believed that solid alloys of tin with Ge or Si could not be grown successfully because solid solutions of tin with the other Group IV elements exhibit segregation when cooled from the melt.